[clang][deps] Remove hard-coded resource directory from tests

Dependency scanner test for resource directory deduction doesn't account for LLVM builds with custom `CLANG_RESOURCE_DIR`.

This patch ensures we don't hardcode the default behavior into the test and take into account the actual value. This is done by running `%clang -print-resource-dir` and using that as the expected value in test assertions.
New comment also clarifies this is different from running that command as part of the dependency scan.

Reviewed By: mgorny

Differential Revision: https://reviews.llvm.org/D115628